In a continuing investigation of fetal muscle metabolism we are studying metabolic control mechanisms in skeletal muscle from the rhesus fetus. We have determined the tissue levels of the metabolic intermediates and cofactors of the glycolytic pathway and calculated the mass-action ratio for each reaction. At midterm, the apparent equilibrium constants in fetal muscle were over 800 times larger than the mass-action ratios for hexokinase, phosphofructokinase and pyruvate kinase, evidence that these three enzymes can be regulatory early in development. Additional evidence was obtained that fetal phosphofructokinase was rate-limiting by midterm. The activity of PFK was 3-5 times greater in fetal (midterm) than in adult muscle. Cyclic AMP was shown to be highest in fetal muscle at about 50 percent of gestation; the levels at 91 percent of term were also higher than those in the adult muscle. PGE2 (2.8 micron M) stimulated cAMP accumulation in the fetal muscle. Total cAMP phosphodiesterase activity of rhesus skeletal muscle was highest in the 100-day fetus, decreased near term and was lowest in the adult. Kinetic data indicated the existence of two cAMP phosphodiesterase enzymes in both the fetal and adult muscle. The apparent Km values for the high-affinity phosphodiesterase enzyme were similar in the 100-day fetal and adult muscle, whereas those for the low-affinity enzyme were 2-fold higher in the fetal series. The Vmax of the high Km enzyme was 10-fold higher in the fetal than in the adult muscle and the low Km Vmax was 4-fold higher in the fetal muscle. BIBLIOGRAPHIC REFERENCES: Bocek, R.M., Young, M.K. and Beatty, C.H. Cyclic AMP in developing muscle of the rhesus monkey: Effect of prostaglandin E2. Biol. Neonate 28:92-103, 1976. Beatty, C.H. and Bocek, R.M. Metabolic aspects of fetal skeletal muscle. In: Fetal and Postnatal Cellular Growth: Hormones and Nutrition, ed., Donald B. Cheek, John Wiley & Sons, N.Y., 1975, pp. 257-271.